[FEA] Parquet reader: replace skip_rows / num_rows with start_row / end_row

Our external interface to the parquet reader allows the user to specify `skip_rows` / `num_rows` parameters when calling it. Internally, we use the same values. But it is a very unwieldy way to think about things. I think it would be easier to immediately convert those values to `start_row` and `end_row` and use that everywhere. It's a nontrivial amount of work to do this without causing bugs but I think the code would be more natural (in the std::algorithms / iterator sense of the word).